I found this and it would appear that they cannot do that:
https://www.nlrb.gov/news-outreach/news-story/board-rules-that-employers-may-not-offer-severance-agreements-requiring
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It's been exactly how it is, just ask T-Mobile and ALL other fortune 500 companies.
Well we are not in the public sector, this is the private sector...so it should apply
"To be clear, the NLRA and this ruling apply to both union and non-union workers. However, the NLRA doesn’t protect workers in the public sector or those in management positions, so the new ruling wouldn’t apply to their severance agreements."
It may or may not be legal but fear is a great motivator. I am 100% sure that this decision was thoroughly reviewed by legal and any loophole available was used.